I have a NA 5.0 with shorty headers, BBK X pipe, and 40 series Flow Masters. The exhaust is very aggressive sounding but below 2500 RPM the exhaust drones. It will give anyone a headache in just a few minutes.
I need a fix and am hopeful it will only require different mufflers. What are ya'll guys experience and/or sudjestions?

I had Flowmaster Super40s on my Miata when I bought it but the drone - esp with the top up - was impossible to live with. I swapped them out for Flowmaster dBX mufflers and problem solved. Sound is still awesome IMHO.
Hear it here

I had unequal length resonators put on one of mine to solve the issue. The different lengths made each pipe "sing" at a slightly different pitch which kept them from droning (think two baritones singing in harmony instead of in unison). On my last one I had no resonators and Borla ProXS mufflers-also no drone but way too quiet until you stepped on it. I added a set of dumps for the track so I'd be at least as loud as the turbo Miatas.
-Jason

The drone can be addressed with some creative solutions like cited above; however, The best solution imho is to pick a muffler that does not drone. The flowmasters and magnaflows sound mean (I.e., LOUD) but they drone. The most common reasonably priced mufflers that I have found that do not drone are the dynomax super turbo and the Borla Pro XS.

I now use the Borla Pro XS on all of my mustangs and will be switching the magnaflows that are on my Miata to the Borla soon as well. Exhaust note is a personal preference for sure but I like the deep, low tone of the Borla without being raspy or overly loud at idle or cruise. I drive mine a LOT and like it "quiet" at idle and cruise...but When you stand on it the exhaust note increases nicely.